That error code happens when in the 8 seconds of upload testing we see less than 2 samples, we should see around 20 to 40 samples.
This suggests something is wrong with the upload side of things, i.e. combined inference from poor upload connection speed and the speed test oddity.
Time to chase EE support after you've checked that switching to the test socket to see if this improves the upload and ensuring all the other sockets in the home don't work.
Does pressing submit on things like searches and other interactivity on the internet feel a bit slow?